What are surfaces and metaballs used for?
I can't speak for metaballs, but surfaces are the geometry you would use before computers could handle smoothing polygon models. The idea of a nurbs surface is that you have a number of isoparms, which are the control points, much like a lattice. The computer interpolates smoothly between adjacent isoparms, which generates the smooth surface you see in the viewport. It was a very efficient way to render a surface with the hardware of the time. 

Today, surfaces can be used for hard surface modeling, ie cars, planes and environments, although their use is fading quickly.